I'm not at liberty to say which companies are using Rust without asking their permission, but one enormous company that's been publicly open about their usage of Rust is Dropbox. To avoid divulging anything myself, I'll just link to what they've posted elsewhere:

https://www.reddit.com/r/programming/comments/374mre/we_just...

https://www.reddit.com/r/rust/comments/2xvtll/getting_acquai...

https://www.reddit.com/r/rust/comments/387ucr/question_are_e...

Having met them for lunch a few weeks ago, I can confirm that this project is still going strong. :)

EDIT: And if you were asking for the names of any startups using Rust, our next SF meetup has the theme "Rust in Production", with presentations by Tilde, MadeSafe, Ironworks, and Terminal.org: http://www.meetup.com/Rust-Bay-Area/events/222260315/